The present invention relates to wrap film for domestic and commercial use and, more particularly, to wrap film satisfactory in safety, saw blade cutting properties, cling, transparency, tear resistance, and unwinding properties, and suitable to microwave heating, refrigerating, and freezing. The present invention also relates to a biaxially stretched polyolefin multilayer film which is excellent in unwinding properties, cutting properties, cling, and heat resistance, generates little harmful gas on combustion, and economically provides wrap films, such as a household wrap film.
Wrap films for home use or commercial use which are typically used to overwrap foods mainly comprise polyvinylidene chloride (PVDC), polyvinyl chloride (PVC), polyethylene (PE) or polypropylene (PP).
Stretched film mainly comprising PVDC which is widespread as household wrap film has satisfactory clinging properties and saw blade cutting properties in good balance but, because of its high tensile strength, lacks flexible deformability to cling in conformity to the shape of a container as obtained with stretch wrap film for commercial use hereinafter described. While it exhibits relatively satisfactory cling to objects with high surface smoothness such as glass, it fails to cling to containers which do not have high surface smoothness such as resins and metals with such intimate contact that a liquid in the container may not leak or spill.
Stretch wrap films for commercial use are flexible films including a single layer film of PVC and a single layer or multilayer film comprising PE, PP or an ethylene-vinyl acetate polymer. These films are easily conformed to the shape of containers because of their low tensile strength to achieve cling such that a liquid in the container may not leak or spill. Having a high elongation at break in nature of use, however, they are hardly cut or do not cut sharply with a safe saw blade used for wrap film for domestic use. PE or PP wrap films designed for domestic use are not convenient to use due to insufficient contact and cutting properties.
Containing chlorine, PVDC and PVC can generate dioxins on incineration, which has given rise to a serious environmental issue. PVC wrap films suffer whitening on contact with boiling water. PVDC wrap films undergo considerable shrinkage on heating so that they can tear in microwave cooking. PE wrap films have insufficient heat resistance so that they can break due to melting or fusion to each other or melt to adhere to a container in microwave cooking. It has been proposed to use PP as a film material free of these problems. However, the state-of-the-art PP wrap films lack cling to a container or self-cling and also have unsatisfactory cutting properties.
It has been proposed to add a tackifier such as polybutene into PP wrap films to impart cling or to stretch the film to improve cutting properties. For example, JP-A-58-65632 discloses a technique for providing a food wrap film having improved self-cling, heat resistance, transparency and cutting properties, in which a polypropylene resin composition is stretched at least uniaxially to an areal stretch ratio of 1.3 or more at a temperature higher than the glass transition point by more than 30° C. The compounding additives referred to in the disclosure are only heat stabilizers and antistatic agents, and one cannot tell how these additives are influential on self-cling or transparency.
JP-B-2-14935 teaches a method for providing a wrap film with improved self-cling, cutting properties and transparency, in which a film of a polypropylene resin composition containing a tackifier having been stretched at least transversely is subjected to surface oxidation and then heat treatment at a temperature not lower than 50° C. and not higher than the crystal melting point, and, after winding in roll form, aged at 60 to 120° C. for 30 minutes or longer. According to this technique, the treatments make the tackifier bleed uniformly, which is taken advantage of to develop self-cling and transparency. However, the wrap film obtained by this method is still unsatisfactory in intimate contact (self-cling) and unwinding properties (ease of feed from a box in which a roll of the film is contained).
Apart from the performance as a wrapping material, chlorine-free wrap films mainly comprising polyethylene resins have been developed to solve the problem of dioxin generation from post consumer plastic films. However, having poor cutting properties and low heat resistance, many of the PE wrap films currently available do not command general consumer acceptance.
In order to improve cutting properties and heat resistance, multilayer wrap films mainly comprising various chlorine-free polyamide resins or polyester resins have been suggested. For example, JP-A-8-80600 discloses a laminate wrap film excellent in cutting properties, antifog properties, transparency, and heat resistance which comprises polypropylene resin surface layers and a polyamide resin intermediate layer. JP-A-5-116262 discloses a laminate wrap film excellent in cutting properties, heat resistance, transparency and environmental friendliness which comprises a polypropylene as an outer layer and an inner layer and a polyester resin as an intermediate layer.
The wrap films having a multilayer structure have been succeeding in improving cutting properties and heat resistance but still have room for improvement on workability and economy in the production. That is, a laminate wrap film having a polyamide resin or a polyester resin as an intermediate layer incurs a material cost increase because these resin materials are more expensive than polypropylene resins. In industrial production, these resins are liable to soil a die for forming a multilayer sheet. The laminate should be provided with an adhesive layer for preventing delamination, and a stretching machine also requires a contrivance for preventing delamination. Post-industrial scrap generated from residuals, such as trimmings and slitting loss, of such films comprising different resin compositions are difficult to reclaim.
Thus, laminate wrap films whose surface layers and intermediate layer have different resin compositions cannot be seen as satisfactory from the aspect of workability and economy in the production. It has therefore been keenly demanded to develop an inexpensive multilayer film which can be produced economically and with good workability.